The illumination of recreational and sports facilities and venues typically involves the use of directional floodlights mounted on tall structures such as poles, buildings etc. An array of such floodlights is employed to provide a uniform intensity of light across key areas such as playing fields or recreational surfaces where bright, uniform illumination is required.
Luminaires utilizing arc type light sources in conjunction with parabolic reflectors are commonly used in such applications, particularly outdoors. Typically such light sources are designed with the axis of the arc lamp coaxially aligned with the axis of symmetry of the reflector. However, when a metal halide arc lamp is aimed at a specific point on a playing field or other target surface it experiences reduced light output due to the well known "tilt factor", by which the efficiency of the lamp is reduced when the lamp is tilted out of its optimal designed operating range. With an arc lamp that is coaxially aligned with the axis of the reflector, tilting the reflector toward the target surface necessarily results in tilting of the arc lamp, which accordingly reduces the light output of the lamp.
It is known to provide such a luminaire with the axis of the arc lamp offset from the axis of the reflector, as for example the floodlight described in U.S. Pat. No. 5,016,150 issued May 14, 1991 to Gordin et al., which is incorporated herein by reference. The luminaire taught by Gordin et al primarily utilizes a conventional hemispherical or truncated hemispherical reflector in which the arc lamp is oriented horizontally when the reflector is directed at a downward angle. However, this presents a problem in the efficiency and uniformity of the light distribution pattern produced by the floodlight.
Unlike a conventional incandescent lamp, an arc lamp does not radiate light uniformly about the light source. The arc tube is elongated and radiates light of greater intensity through the elongated wall of the tube and relatively poorly approaching the ends of the tube. This is suitable when the lamp is oriented coaxially with the reflector, because a conventional reflector is also designed symmetrically about the wall of the arc tube. However, when the arc tube is mounted in a conventional reflector with its axis offset from the reflector axis, the light distribution pattern produced by the floodlight is no longer uniform.
U.S. Pat. No. 5,016,150 to Gordin et al illustrates in one embodiment a horizontally mounted arc lamp with an asymmetric reflector having an elongated top portion, designed to position the arc tube generally photometrically centered within the reflector. The asymmetric reflector configuration increases the manufacturing cost of the reflector, and can present problems in the alignment of the arc lamp, often requiring a specialized lamp base to ensure that the lamp is properly positioned within the reflector.
Further, mounting the arc lamp horizontally causes undue strain on the neck of the lamp, particularly at the point of attachment to the base. The weight of the bulb and the arc tube is transferred as a torque to the neck, the torque being concentrated about the top of the neck rather than being evenly distributed about the circumference of the neck. Such arc lamps can be fairly heavy, and when subjected to high winds have been known to break at the lamp base for this reason.